Phase-Shift Fiber-Loop Ring-Down Spectroscopy

Abstract

Fiber-loop ring-down spectroscopy (FLRDS) is a recently developed absorption spectroscopic technique suitable for very small liquid samples. It is based on measurements of the optical decay constant of laser intensity in a loop made of optical waveguide material. This decay constant changes as small liquid samples containing absorbing species are introduced into the loop. In this report, it is demonstrated that one can also obtain the optical decay constant using a continuous wave laser beam that is intensity modulated and then coupled into an optical fiber loop. The inherent exponential decay in the fiber loop introduces a phase shift of the light emitted from the loop with respect to the pumping beam. By measuring this phase shift, one can readily determine the concentration of the analyte introduced between the two fiber ends and a model is established to describe the relationship. It is demonstrated that this technique, dubbed phase-shift fiber-loop ring-down spectroscopy (PS-FLRDS), is well suited as an absorption detector for any flow system in which the optical absorption path is limited by the instrument architecture. By measuring the phase angle as a function of concentration of 1,1'-diethyl-4,4'-dicarbocyanine iodide in dimethyl sulfoxide, the detection limit was determined as approximately 6 microM for a 30-40-microm absorption path. A temporal resolution of approximately 100 ms was demonstrated by a rapid displacement of the solutions between the two fiber ends. Proof-of-principle use of the PS-FLRDS detection in capillary flow systems using a commercial four-way microcross established that the alignment of the fiber and the capillary can be made simple and effective, while retaining both a low detection limit and a fast response.

Detection of volatile organic compounds with functionalized long-period gratings and micro-ring resonators

“…Cavity ring-down (CRD) spectroscopy has been used in recent years to obtain the optical loss in fiber optic cavities without the need for reference intensity measurements [5]. Here, we implement fiber-loop (CRD) spectroscopy in its phase-shift CRD variant ( Fig 1A) [6]. Details on the optical design have been published elsewhere [2].…”

Liquid-Phase and Evanescent-Wave Cavity Ring-Down Spectroscopy in Analytical Chemistry

“…Because of the small dimensions of fiber cores, the volumes of these detection cells can be made as small as several picoliters, thereby allowing for CE detection (78). The principle of liquid-phase fiber-loop systems can be combined with various modes of CRDS, such as phase-shift CRDS (79). In this mode, the phase-shift of a sinusoidally intensity-modulated CW laser (rather than a ring-down transient) is measured after stopping the excitation of the optical resonator.…”

“…Because the absorption measurement is based on the phase-shift of this integrated light, phase-shift CRDS is less sensitive to light-source-intensity fluctuations than is CEAS. LODs using the phase-shift fiber-loop CRDS are on the order of several micromolars for a strong absorber-a 45-fold improvement over the pulsed fiber-loop CRDS setup, where a flow-cell with a 10-times-shorter optical pathlength was used (77,79).…”

Cavity Ringdown Laser Absorption Spectroscopy

“…The same group further advanced the fiber loop ringdown technique by introducing phase-shift measurement, which greatly improved the data-acquisition rate to close to real time (10-100 ms). (60) This technique has been demonstrated to be suitable for low-cost, real-time, and on-line detection of capillary electrophoresis with a detection limit at micromolar concentration levels. Using flow injections, the device can detect a series of solution samples at different concentrations.…”